diff --git a/src/lib/offline/sync.test.ts b/src/lib/offline/sync.test.ts
new file mode 100644
index 0000000..7a4489b
--- /dev/null
+++ b/src/lib/offline/sync.test.ts
@@ -0,0 +1,64 @@
+// @vitest-environment jsdom
+// src/lib/offline/sync.test.ts
+import { describe, it, expect, beforeEach, vi } from "vitest";
+import "fake-indexeddb/auto";
+import { enqueueAction } from "./queue";
+import { syncPending, calculateBackoff, BACKOFF_SCHEDULE, MAX_ATTEMPTS } from "./sync";
+
+beforeEach(async () => {
+  const { getOfflineDB } = await import("./db");
+  const db = await getOfflineDB();
+  await db.clear("queue");
+});
+
+describe("offline sync — backoff", () => {
+  it("BACKOFF_SCHEDULE returns 1s, 4s, 16s, 60s, 300s", () => {
+    expect(BACKOFF_SCHEDULE).toEqual([1000, 4000, 16000, 60000, 300000]);
+  });
+
+  it("MAX_ATTEMPTS is 5", () => {
+    expect(MAX_ATTEMPTS).toBe(5);
+  });
+
+  it("calculateBackoff(0) returns the first delay", () => {
+    expect(calculateBackoff(0)).toBe(1000);
+  });
+
+  it("calculateBackoff(4) returns the last delay", () => {
+    expect(calculateBackoff(4)).toBe(300000);
+  });
+
+  it("calculateBackoff(99) caps at MAX_ATTEMPTS (gives up)", () => {
+    expect(calculateBackoff(99)).toBe(300000);
+  });
+});
+
+describe("offline sync — replay", () => {
+  it("syncPending posts each action and marks it synced on success", async () => {
+    const a = await enqueueAction("markOrderReady", { orderId: "abc" });
+    const dispatcher = vi.fn().mockResolvedValue({ ok: true });
+    const result = await syncPending(dispatcher, { online: true, sleep: () => Promise.resolve() });
+    expect(dispatcher).toHaveBeenCalledWith("markOrderReady", { orderId: "abc" }, a.id);
+    expect(result.synced).toBe(1);
+    expect(result.conflicts).toBe(0);
+    expect(result.failed).toBe(0);
+  });
+
+  it("syncPending marks action as conflict when dispatcher returns conflict", async () => {
+    await enqueueAction("markOrderReady", { orderId: "abc" });
+    const dispatcher = vi.fn().mockResolvedValue({ ok: false, conflict: "stale" });
+    const result = await syncPending(dispatcher, { online: true, sleep: () => Promise.resolve() });
+    expect(result.conflicts).toBe(1);
+    const { getQueuedActions } = await import("./queue");
+    const all = await getQueuedActions();
+    expect(all[0].status).toBe("conflict");
+  });
+
+  it("syncPending skips when offline", async () => {
+    await enqueueAction("markOrderReady", { orderId: "abc" });
+    const dispatcher = vi.fn();
+    const result = await syncPending(dispatcher, { online: false, sleep: () => Promise.resolve() });
+    expect(dispatcher).not.toHaveBeenCalled();
+    expect(result.synced).toBe(0);
+  });
+});
diff --git a/src/lib/offline/sync.ts b/src/lib/offline/sync.ts
new file mode 100644
index 0000000..387e422
--- /dev/null
+++ b/src/lib/offline/sync.ts
@@ -0,0 +1,86 @@
+// src/lib/offline/sync.ts
+import { enqueueAction, getQueuedActions, markInFlight, markSynced, markConflict, markFailed } from "./queue";
+
+export const BACKOFF_SCHEDULE = [1000, 4000, 16000, 60000, 300000] as const;
+export const MAX_ATTEMPTS = 5;
+
+export function calculateBackoff(attempt: number): number {
+  const idx = Math.min(attempt, BACKOFF_SCHEDULE.length - 1);
+  return BACKOFF_SCHEDULE[idx];
+}
+
+export type Dispatcher = (actionName: string, payload: unknown, clientActionId: string) => Promise<
+  | { ok: true }
+  | { ok: false; conflict: string }
+  | { ok: false; error: string }
+>;
+
+export interface SyncOptions {
+  online: boolean;
+  sleep: (ms: number) => Promise<void>;
+  now?: () => number;
+}
+
+export interface SyncResult {
+  synced: number;
+  conflicts: number;
+  failed: number;
+}
+
+export async function syncPending(dispatcher: Dispatcher, options: SyncOptions): Promise<SyncResult> {
+  const result: SyncResult = { synced: 0, conflicts: 0, failed: 0 };
+  if (!options.online) return result;
+
+  const actions = await getQueuedActions();
+  for (const action of actions) {
+    if (action.status === "conflict" || action.status === "failed") {
+      // Don't auto-retry conflicts/failures; require manual user intervention
+      continue;
+    }
+    if (action.attempts >= MAX_ATTEMPTS) {
+      await markFailed(action.id, "max attempts reached");
+      result.failed += 1;
+      continue;
+    }
+
+    const backoff = calculateBackoff(action.attempts);
+    if (action.lastAttemptAt) {
+      const elapsed = (options.now?.() ?? Date.now()) - action.lastAttemptAt;
+      if (elapsed < backoff) continue; // not time yet
+    }
+
+    await markInFlight(action.id);
+    try {
+      const dispatchResult = await dispatcher(action.actionName, action.payload, action.id);
+      if (dispatchResult.ok) {
+        await markSynced(action.id);
+        result.synced += 1;
+      } else if ("conflict" in dispatchResult) {
+        await markConflict(action.id, dispatchResult.conflict);
+        result.conflicts += 1;
+      } else {
+        await markFailed(action.id, dispatchResult.error);
+        result.failed += 1;
+      }
+    } catch (err) {
+      await markFailed(action.id, (err as Error).message);
+      result.failed += 1;
+    }
+  }
+  return result;
+}
+
+/**
+ * Wire this up to navigator.onLine + 'online' event in the client.
+ * Returns an unsubscribe function.
+ */
+export function attachSyncListener(dispatcher: Dispatcher): () => void {
+  if (typeof window === "undefined") return () => {};
+  const handler = () => {
+    void syncPending(dispatcher, { online: navigator.onLine, sleep: (ms) => new Promise((r) => setTimeout(r, ms)) });
+  };
+  window.addEventListener("online", handler);
+  // Also fire once on attach if online
+  if (navigator.onLine) void handler();
+  return () => window.removeEventListener("online", handler);
+}